1. Field of the Invention
The invention relates to an optical rotation angle polarimeter, more particularly to a noninvasive optical rotation angle polarimeter, which can be used to investigate the optical activity of optically active materials or substances.
2. Description of the Related Art
Polarizers and analyzers, in combination with mechanical methods for generating intensity-modulated signals, have been used to measure directly the optical rotation angle introduced by an optically active material or substance on linear polarized light to determine a physical property of the optically active material or substance, such as the refractive index of solid optically active materials or the concentration of optically active substances in a solution. Because of the susceptibility of the intensity-modulated signals generated by conventional mechanical methods to vibrations, and the relatively low frequency of the intensity-modulated signals that make it difficult to achieve high measuring precision, and because of the relatively slow measuring speed that results when conventional mechanical methods are in use, the conventional mechanical methods are currently being replaced with optic modulators, such as acousto-optic, electro-optic or photoelastic modulators, as intensity-modulated incident light sources. With the use of optic modulators, the magnitude of the optical rotation angle can be obtained by measuring directly the amplitude or phase of an intensity-modulated output light signal. However, because optic modulators are unstable, the high frequency intensity-modulated signals or polarization modulation signals generated thereby are prone to errors. Although a relatively high modulating frequency and relatively high stability can be achieved when optical heterodyne interference is generated in conjunction with acousto-optic modulators, the interferometer is very sensitive to disturbance from the outside environment, which can lead to limitations in the measuring precision of the system.
In co-pending U.S. patent application Ser. No. 08/967,691, there is disclosed a method and apparatus for determining the concentration of an optically active substance in a medium. In the disclosed method, a laser beam with two eigen modes of two different frequencies and two orthogonal linear polarized states is generated and passed through the medium. Then, the laser beam that exits the medium is passed through an analyzing polarizer so as to generate an optical heterodyne of the orthogonal linear polarized states. The amplitude of the optical heterodyne is detected for subsequent conversion into the concentration of the optically active substance in the medium. The method and apparatus are suitable for noninvasive in vivo glucose monitoring of the aqueous humor in an eye of an animal.